For people trying to maximize points and miles earned on every dollar spent or meet a large minimum spending requirement, a lot of their biggest expenses don't accept credit cards for payment. That's spend that is not earning you any bonuses or rewards back. Plastiq is a bill pay service that is useful for bills or expenses that don't otherwise accept credit cards or otherwise charge a larger fee. Plastiq charges a 2.5% transaction fee for all credit card payments, and can be used to pay most bills.
(Visa and Amex don't allow their cards to be used on Plastiq to pay for mortgages, and some people have reported trouble getting car payments to go through. However, there are still gaps in these restricted kind of payments, like using
@vanjust14 's trick to get around Plastiq's blacklist to pay her mortgage company.)
In most cases, yes, Plastiq's 2.5% fee would be a major cost that would eat into the credit card rewards and bonuses. However, it makes sense to use Plastiq in two situations:
1.
When you pay bills through Plastiq using the CIP set as the payment method. Bills paid through Plastiq with the CIP currently earn 3x UR on the principal amount and the fee, thereby negating the fee.
So if I have a bill for $1,000 and pay a 2.5% fee of $25, I'll earn 3x UR back on that $1,025 when I use my CIP, earning 3,075 UR points, which is worth at least $30.75 if I decided to cash out the points or receive a statement credit where 1 UR = 1¢. This is like buying 1 UR, which is worth 1¢, for 0.81¢. If I transfer the 3,075 UR points to my CSR account, they are worth 50% more, or $46.12, towards travel booked on the Chase UR portal.
I think the CIP is the only card that still earns 3x points on Plastiq. Keep in mind that Plastiq could change how it codes payments on the CIP at any time, and has in the past. Last year, in early summer, the CSR and CIP earned 3x UR for paying rents and mortgages through Plastiq, because Plastiq was coding these payments as lodging and therefore earned 3x under travel. That came to an end after about 3 months. Then the CIP started earning 3x UR on, I believe, all kinds of payments (I earn 3x on my rent, utilities, student loans and DD's tuition with CIP through Plastiq). It's still coding this way, so get in while the gettin's good. (Plus you have a minimum spend to meet on the CIP, right?)
2.
When you are trying to meet a minimum spending requirement on a new credit card with a big bonus. Some cards carry big bonuses, but also large spending requirements. For example, if you got the Amex Business Rewards Gold with a 75,000 MR points signup bonus, but you don't think you can meet a $5,000 or $10,000 spend in 3 months, rather than give up on the signup bonus that's worth at least $750, you start to think how much you might fall short, and what bills might not otherwise take credit cards but you can move over to Plastiq. Say that $1,000 bill from above would help you meet the minimum spend on your new Amex card. You'll pay the $25 fee to Plastiq, and it won't earn you 3x back like on the CIP, but it will have helped you get a bonus worth much more than what you paid in fees.
ETA: I should have said three situations. Plastiq occasionally has promotions that reduce the fees when you pay with certain brands of credit cards, give you rebates on certain kinds of payments, give you a "fee free" month if you schedule recurring payments for a certain amount of time, and "fee free dollars" or no fees on like $1000 when you refer new users. These all would lessen the cost of the promoted kinds of payments through Plastiq.